Two output characteristics are usually described for an optical transmission system: the transmission rate, e.g. in Mbit/s as a measure of the amount of transmitted data, and the system range, which indicates the maximum attenuation that may be placed between emitter and receiver, if a certain quality of transmission is to be ensured.
However, further information is needed for practical use. A receiver is only able to function optimally within a certain range of the optical input level. Too low a radiation capacity, as well as too high a level, can impair the transmission quality. The path attenuation in optical transmission systems is a function of fiber length and the fiber attenuation coefficient. In addition, emitter output and receiver sensitivity have tolerances and may exhibit aging. For these reasons, a component for adapting the path attenuation to the receiver's optimum function range is needed. So-called attenuation devices are used for this purpose.
An attenuation adapter is known for the above named purpose, and is inserted as a plug or as an intermediate connector into the transmission path. This plug connector has a certain attenuation value.
An alternative solution is a variable optical attenuation device, which makes several graduated attenuations possible, and is directly installed on the receiver plate, between the plug connector and the detector.
A requirement of such attenuation devices in single mode fiber transmission systems is that they be designed to have the lowest reflection possible. Direct reflection back to the laser could lead to feedback noise and deteriorate the transmission quality considerably.
Reflections are avoided in an attenuation device configured as a plug connector, by grinding the plug connectors at an angle.
A filter magazine with different neutral filters is installed in the beam inlet of the variable attenuation device, which has been expanded by spherical lenses. Reflection is prevented in this instance by inclining the filter disks at an angle of about 1.5.degree. with respect to the optical axis. These attenuation devices contain mechanically adjusted parts, which can only be made to be unaffected by environmental influences at great expense (NTZ Vol. 39 (1986), booklet 7, page 481-482).